unbreak (future) Mac OS 10.9, 11.0, ... builds

Disappeared syscalls are better 
  #ifdef-ed with "< version_in_which_they_disappear"

Newly appeared syscalls are better
  #ifdef-ed with ">= version_in_which_they_appear"



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13669 a5019735-40e9-0310-863c-91ae7b9d1cf9